The present invention relates generally to fiber optic optical memory devices, and particularly to in-line fiber optic memories which recirculate optical signals, and which utilize non-linear effects for amplification of the recirculating signals.
Fiber optic recirculating memories typically comprise a loop of optical fiber and a fiber optic directional coupler for coupling light to and from the loop of optical fiber. An exemplary fiber optic recirculating memory is disclosed in U.S. Pat. No. 4,473,270, issued Sept. 25, 1984, which is incorporated by reference herein. As disclosed in that patent, a single signal pulse is supplied as an input to the memory device. This pulse recirculates in the loop and a portion of the pulse is coupled out of the loop on each circulation to provide a series of output signal pulses identical to the input signal pulse, although at smaller, gradually decreasing amplitudes. Such a device is particularly useful, for example, to provide a short term optical memory in a system where data is generated at a rate faster than it can be accepted by a data processor, or to adjust time delays between systems exchanging data (optical delay line) or to repeat data (data regeneration). This device may also be utilized as a pulse train generator, or as a filter. Moreover, recirculating memory devices may be utilized as re-entrant fiber sensing loops for fiber optic gyroscopes.
Although the recirculating memory disclosed in the above-referenced patent represents a significant advance in the art, and is quite advantageous for many optical memory applications, the optical memory capabilities of such device are limited due to the fact that the circulating signal pulse is degraded by coupling a portion of this pulse on each circulation around the fiber loop to provide the train of output pulses. Further, fiber propagation losses attenuate the optical signal pulse as it propagates through the loop, causing additional losses. These losses cause the intensity of the signal pulse circulating in the loop to decay, which causes a concomitant decay in the series of output pulses, thereby limiting the number of useable output pulses and the lifetime of the optical signal memory.
In order to prevent the problems associated with decay of the circulating pulse, it has been proposed in the prior art to insert a fiber optic amplifier in the loop so as to amplify the pulse on each circulation to compensate for the loop losses. Such a system, for example, has been disclosed in U.S. Pat. No. 4,136,929, to Suzaki, entitled "APPARATUS FOR GENERATING LIGHT PULSE TRAIN". Fiber optic amplifiers, however, are typically discrete components, which must be spliced into the fiber loop. Such splicing will cause additional loop losses which must be compensated for by the fiber optic amplifier.
Accordingly, there is a need in the art for a low loss in-line recirculating optical memory which utilizes an amplification process capable of providing sufficient gain to compensate for the total round trip loop losses.